1. Field of the Invention
This invention relates to an apparatus for detecting a focus error signal for recording, reproducing or erasing information by means of a laser beam and, more particularly, to a focusing servo device for controlling a spot of a laser beam by adjusting a position of an objective lens by means of the reflected laser beam from a magnetooptical disk (referred to as MO disk hereunder).
2. Related Art
As optical recording mediums, a laser video disk (LVD), a digital audio disk (DAD), a compact disk read only memory (CD-ROM) and an MO disk memory are conventionally known. An optical head is used for reading information from these disks and writing information onto these disks. The optical head emits a laser beam to read information recorded on a disk and write information onto the disk. The laser beam must be emitted accurately onto the information recording surface of the disk. To this end, the optical head is usually controlled by a focusing servo device and a tracking servo device.
A conventional focusing servo device converts a beam reflected by the information recording surface of the disk into electric signals and controls an actuator for focusing operation so that focus error signals among the electric signals become naught. As a method of the focusing servo operation, a method utilizing astigmatism is known. A focusing servo device which adopts this method comprises a cylindrical lens for generating astigmatism and a quadrant type photodetector having four light receiving regions. The cylindrical lens generates two elliptic light spots on an optical path. The quadrant photodetector is disposed in such a manner that its light receiving surface divided into four regions or sections is located at a predetermined position where a truly circular light spot of the laser beam is reflected on the light receiving surface when the laser beam is focused correctly on the disk. If the focal point is deviated forward or backward from the predetermined position, an elliptic light spot is formed on an optical path. The quadrant photodetector is disposed in such manner that in case that the focal point of a laser beam is deviated from a pit of the disk along its optical axis, an elliptic light spot is formed in the direction of a diagonal line passing through the first and third quadrant regions or the second and fourth quadrant regions. An electric output corresponding to a difference in quantity of received light between two regions forming one diagonal pair and two regions forming the other diagonal pair is outputted. In this case, if the difference in quantity of received light is not zero, the focal point does not coincide with the recording surface of the disk. A focusing servo control is performed so that the difference in quantity of received light becomes naught.
The accuracy of the focusing servo control has a connection with the optical property of material of a substrate with which the recording surface of the disk is covered. As a material of a disk substrate, glass or polymethyl methacrylate resin (PMMA) is often used in view of its optical property such as transparency. However, in view of cost and mass-production, it is considered that polycarbonate resin is used because polycarbonate is more low-priced than PMMA resin. Polycarbonate resin has a high double refractive index as an optical property. If the material of a disk substrate has a high double refractive index, the material has the following bad effect upon the focusing servo control in the recording or reproduction magnetooptically. That is, a laser beam is converged in the shape of a cone while passing through the substrate of the disk. At this time, rays near its optical axis enter perpendicularly the recording surface of the disk, and, however, rays at an outer periphery of bundle of rays enter there with a large angle of incidence. In this case, if polycarbonate resin having a high double refractive index is used for a substrate with which the recording surface of the disk is covered, the rays at outer periphery are double-refracted between a point of incidence and a reflection point. This double reflection or the birefringence is an optical phenomenon that two refracted ray appear when the light enters the anisotrophic medium such as the crystal. This birefringence causes a phenomenon that the phase of the rays at the periphery of the bundle of rays is delayed in comparison with rays near the center portion of its optical axis. This phenomenon is called "retardation". As a result, rays at a periphery of bundle of rays are elliptically polarized. There appear some uneven parts each having a high intensity of light at a periphery of a light spot formed on the light receiving surface of the photodetector due to the elliptic polarization of light and the use of the polarization beam splitter (referred to as PBS hereunder) in the magnetooptical head. If there appear a few of high intensity parts at two divided regions (e.g. two regions of the first and third quadrants), this causes a state wherein there is a difference in quantity of received light between two diagonal pairs of regions of the photodetector in spite of a good focusing. With this state, the focal point of an objective lens is moved needlessly, and focusing servo control does not operate in case of need.